Method and Apparatus for Liquefying a CO2-Rich Gas

ABSTRACT

This invention relates to a method for liquefying a gas containing at least 60 mol % of CO2, in order to produce at least one liquid product, wherein the gas is cooled in order to form a fluid flow, at least a portion of the liquid or supercritical flow is cooled in a heat exchanger in order to form a cycle fluid having a cycle pressure, the cycle fluid is divided into at least two fractions including an auxiliary fraction, one of the fractions being expanded up to a first pressure in a valve in order to form a biphasic mixture, and then sent to a phase separator. The liquid fraction of the phase separator is vaporized so as to form a vaporized gas in the exchanger, the vaporized gas then being expanded from the first pressure to a second pressure in an expansion means, and then compressed in the cycle compressor and mixed with the first feed gas.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a §371 of International PCT Application PCT/FR2012/050743, filed Apr. 5, 2012, which claims the benefit of FR 1153128, filed Apr. 11, 2011, both of which are herein incorporated by reference in their entireties.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a process and an apparatus for liquefying a CO₂-rich gas.

SUMMARY OF THE INVENTION

A carbon dioxide-rich gas contains at least 60 mol % of carbon dioxide, or even at least 80 mol % of carbon dioxide.

The gas also contains at least one impurity that is lighter than carbon dioxide, such as nitrogen, oxygen, carbon monoxide, etc.

An industrial means envisaged for transporting CO₂ is that of boat transportation, requiring liquefaction of the CO₂, for example coming from different sources: gas from coal-fired power plants, metallurgical plants, SMR, gasification processes, etc.

It is sometimes necessary to transport CO₂ by pipeline at supercritical pressures, and, for this, the liquid to be transported must be pressurized at high pressures.

This liquefaction can be preceded by one or more fume (or synthesis gas) treatments by physical and/or chemical separation process.

The invention proposes a solution for managing the content variations of the source(s) in CO₂ liquefaction units in order to prevent any risk of freezing of the CO₂ in the refrigeration circuit of the system. It also proposes a system for optimal regulation of rotary machines.

In the diagram, the CO₂-rich gas resulting from one or more sources is compressed via the cycle compressor or is already at the desired pressure for condensing the gas at ambient temperature. The condensed gas is cooled. A portion of this condensed gas can be compressed directly at a pressure sufficient for transporting the CO₂ by pipeline. Another portion of the condensed gas, or all of the condensed gas, perhaps, is used in the cold box. The condensed gas sent into the cold box has two uses: one portion is purified for the production of liquid CO₂, the other provides the refrigerating balance by expansion of the condensed gas at one or more pressures. The condensed and expanded gas is then vaporized and sent to the cycle compressor.

A minimum liquefied gas expansion pressure in the cold box will be determined by the temperature of solidification of the CO₂ contained in the condensed gas, because the presence of solids would damage the system. A temperature margin will be determined so as to prevent freezing in the cold box (−54.5° C. minimum).

It is also traditionally possible to envisage optimizing the energy at the exchanger block by staging the pressure and therefore have different liquefaction levels. It is possible to recycle these flows by introducing them at the inter-stages of the multi-integrated centrifugal cycle compressor.

The composition of impurities in the CO₂-rich gas strongly influences the suction pressures of the cycle compressor. When the contents vary, the suction pressures of the different wheels of the compressor must therefore also be adapted.

The innovative process of the present invention concerns the loop ensuring the refrigeration of the system. The invention therefore involves expanding the impure CO₂ in the cold box to a pressure that enables the refrigerating balance of the system, while preventing freezing (the coldest temperature being limited to −54.5° C.), then re-expanding the gas under heat so as to obtain the same constant suction pressure for the cycle compressor. The latter will moreover be cooled, which will enable a lower energy consumption of the compressor. In this way, the cycle compressor will not be subject to possible suction pressure variations due to a possible variation in the composition of the source.

It is also possible to envisage collecting the additional expansion energy by means of a turbine-compressor arrangement in order to minimize losses, or by means of generator turbines.

The same liquefactor will therefore make it possible to adapt to the case where the content of the source varies and/or the case where a source with a different composition is connected to the liquefactor over time, which would cause the composition of the incoming flow to vary, while ensuring the safety of the installation.

The unit can also comprise the following technological components:

-   -   drying of the gas by adsorption upstream of the compressor,     -   elimination or reduction of impurities such as Hg by adsorption,         NOx via a distillation column,     -   purification of the CO₂ via a distillation column,     -   improvement of the CO₂ yield via intermediate compression in the         cold box.

This arrangement thus enables reliable use of the liquefactor even if one of the CO₂ sources is lost.

If at least one impure CO₂ source is at the right pressure for the liquefaction of the CO₂, the gas from this source is preferentially chosen for the production of liquid CO₂.

Optionally, compression of the gas from one of the sources may be necessary to equalize at least the pressure of the liquid CO₂ produced.

BACKGROUND

Liquefying a CO₂ flow by compressing it and by cooling it according to JP-A-58208117, EP-A-0646756 and SU-A-1479802 is known.

U.S. Pat. No. 4,699,642 describes a process for liquefying a gas containing at least 60 mol % of CO₂ according to the preamble of claim 1.

According to an object of the invention a process is provided for liquefying a gas containing at least 60 mol % and at least one light impurity to produce at least one liquid product wherein a first feed gas containing at least 60 mol % of CO₂ and at least one light impurity at a feed pressure is cooled in order to form a liquid or supercritical flow, possibly after having been compressed in a cycle compressor, at least a portion of the liquid or supercritical flow is cooled in a heat exchanger in order to form a cycle fluid having a cycle pressure, the cycle fluid is divided into at least two fractions comprising an auxiliary fraction, one of the fractions being expanded in a valve down to a first pressure to form a biphasic mixture and sent to a phase separator, the liquid fraction of the phase separator is vaporized to form a vaporized gas in the exchanger, the vaporized gas then being expanded from the first pressure to a second pressure in an expansion means and then compressed in the cycle compressor and mixed with the first feed gas, the auxiliary fraction either comprising the liquid product or one of the liquid products or being treated by separation at sub-ambient temperature in at least one separation means to form the liquid product or one of the liquid products, and wherein the first pressure is varied as a function of the amount of impurities that are lighter than the carbon dioxide contained in the carbon dioxide-rich gas, characterized in that the first pressure is varied so that the higher the concentration of light impurities is, the higher the first pressure is.

According to other optional features:

-   -   the second pressure is substantially constant and independent of         the composition of the carbon dioxide-rich gas,     -   the second pressure is equal to the inlet pressure of the cycle         compressor and the gas expanded in the expansion means is sent         directly from the expansion means to the inlet of the cycle         compressor,     -   the first pressure is chosen to be below the pressure at which         the carbon dioxide-rich gas would solidify in the phase         separator or in the valve,     -   the expansion rate in the expansion means is regulated as a         function of the composition of the gas to be expanded in the         expansion means and/or as a function of the composition of the         fluid expanded in the expansion valve,     -   the expansion rate in the expansion means is regulated as a         function of the quantity of light impurities in the gas to be         expanded in the expansion means and/or as a function of the         composition of the fluid expanded in the expansion valve.

The invention also relates to an apparatus for liquefying a gas containing at least 60 mol % of CO₂ and at least one light impurity in order to produce at least one liquid product, comprising a heat exchanger, a cycle compressor, cooling means, a conduit for sending a feed gas containing at least 60 mol % of CO₂ and at least one light impurity to the cooling means in order to form a liquid or a supercritical fluid, a conduit for sending the liquid or supercritical fluid to the exchanger in order to form a cooled fluid, a conduit for sending a portion of the cooled fluid as an auxiliary fraction to a client or to a treatment, an expansion valve, a conduit for sending a portion of the cooled fluid to the expansion valve, a phase separator, a conduit for sending a fluid expanded in the expansion valve to the phase separator, a conduit for sending the liquid from the phase separator to the exchanger, expansion means, a conduit for sending the liquid vaporized in the exchanger to the expansion means in order to form an expanded gas, a conduit for sending the expanded gas to a cycle compressor, means for mixing the expanded gas with the gas containing at least 60 mol % of CO₂ upstream of, downstream of, or in the cycle compressor and means for varying the outlet pressure of the expansion valve as a function of the composition of the feed gas, characterized in that the expansion means is a turbine.

According to other optional features:

-   -   the apparatus comprises a treatment unit connected to the         conduit to send a portion of the cooled fluid as an auxiliary         fraction to a treatment, the treatment unit being a liquefaction         unit or a separation unit at sub-ambient temperature,     -   the apparatus comprises a means for detecting the temperature of         the fluid to be expanded and/or the temperature of the fluid         expanded in the expansion valve and means for varying the         expansion rate in the expansion valve as a function of this or         these temperature(s),     -   the apparatus comprises a means for detecting the concentration         of the gas sent to the expansion means and/or the fluid sent to         the expansion valve and means for varying the expansion rate of         the expansion means as a function of this concentration,     -   the apparatus comprises means for detecting the pressure of the         gas sent to the expansion means and a means for varying the         expansion rate in the expansion means as a function of this         pressure,     -   the turbine is coupled to the cycle compressor or the treatment         unit comprises a compressor and the turbine is coupled to the         compressor of the treatment unit,     -   the exchanger is arranged so that the latent heat from         vaporization of the liquid of the phase separator is transferred         to the liquid or to the supercritical fluid that is cooled         through the exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.

FIG. 1 shows an embodiment of the present invention.

FIG. 2 shows an embodiment of the present invention.

The invention will be described in greater detail with reference to the figures that show the processes according to the invention.

DETAILED DESCRIPTION

In FIG. 1, the apparatus comprises a heat exchanger E1, a pump 42, a compressor with four stages C1, C2, C3, C4 and a series of phase separators P1, P2, P3.

The phase separators, the heat exchanger and the expansion valves are in a cold box.

A mixture of gas from three different sources is liquefied so as to form a supercritical CO₂ flow and purified so as to form a pure liquid CO₂ flow.

The gas 1, containing at least 60 mol % of CO₂ and at least one light impurity, in this case nitrogen, at a first pressure resulting from a co-generation, is mixed with the gases 1A, 1B and the mixture formed is sent to the third stage C3 of a four-stage compressor. The gas is cooled by the cooler R3, compressed in the fourth stage C4 up to a second pressure beyond the critical pressure, for example 83 bar abs and then cooled in the exchangers E3, E4 so as to form a supercritical fluid. If the pressure is below the critical pressure, a liquid will obviously be formed.

The feed gas(es) can be dried by adsorption upstream of the exchanger E4 or upstream of the compressor.

Optionally, a portion 40 of the fluid is not sent to the heat exchanger E1, but is pressurized by the pump 42 up to a pressure of 150 bar abs in order to form a product, for example to be sent in a pipeline. The rest of the fluid or the fluid at the outlet pressure of stage C4 is cooled liquefied in the exchanger E1 in order to form a supercritical fluid or a cycle liquid. For the present case, it is considered to be a supercritical fluid. The supercritical fluid at 83 bar abs is divided into at least five fractions. A fraction 4 is expanded in the valve 6, up to a very high pressure, cooled in the exchanger E1 and sent to the third stage C4 of the compressor. A fraction 5 is expanded in the valve 15 up to a high pressure, cooled in the exchanger and sent to the third stage C3 of the compressor. A fraction 7 is expanded in a valve 16 up to an average pressure, cooled in the exchanger E1 and sent to the inlet of the second stage C2 of the compressor. A low-pressure fraction 34 is expanded in a valve 43. The mixture formed of gases 1, 1A and 1B contains CO₂ and only nitrogen as a light impurity, but in variable quantities. To ensure that the fraction 34 of supercritical fluid 3 will not freeze while expanding in the valve 43 from 83 bar abs, an expansion pressure is calculated as illustrated in the following table:

mol % N₂ Lowest vaporization pressure in flow 34 abs bar 0.035 5.85 1 7.9 2 8.9

This means that, in the valve 43, the effective expansion of the fluid up to the lowest pressure will be from 83 bar abs to 5.85, 7.9 or 8.9 bar abs, respectively, as a function of the amount of nitrogen in the incoming fluid. The biphasic mixture at the valve outlet 43 is sent to the phase separator 35. The liquid formed 39 and the gas formed 37 are cooled in the exchanger E1 and mixed. The mixture 41 is expanded in a hot valve 45 up to a pressure of 5.85 bar abs, in order to keep a constant suction pressure of the cycle compressor. The inlet pressure of the hot valve 45 is therefore variable since there is neither expansion nor pressurization between the hot valve 45 and the valve 43.

The pressure of this valve outlet 43 is controlled by detecting the temperature of the flow expanded at the outlet of the valve 43 to verify that it is not below −54.5° C. and by detecting the temperature of the sub-cooled liquid 3 in the exchanger E1. The higher the level of non-condensables in the flow 34 is, the more quickly the flow 34 will be cooled with the expansion. The non-condensables are in effect more volatile by definition: therefore they are not expanded to such a low pressure if the flow 34 consists of pure CO₂.

A temperature detection means TIC that measures the temperature of the sub-cooled liquid 3 and a temperature detection means TSLL that measures the temperature of the fluid expanded in the valve 43 can be noted. A low temperature limit not to be exceeded is set and alarms are activated if the temperature goes below a temperature 1° C. above the limit and 0.5° C. above the limit so as to enable the expansion of the valve to be adjusted, as a function of these temperatures. Thus, if a temperature reduction is observed in the fluid at the outlet of the valve 43, which indicates a reduction in purity of the fluid 3, the outlet pressure of this valve will be increased. Similarly, if the temperature of the expanded fluid increases, the outlet pressure of the valve 43 will be lowered.

The hot valve 45 receives gas at different pressures. The expansion rate of this valve is therefore adjusted as a function of the pressure of the flow 41 measured by a PIC analyzer. If the liquid 3 is pure enough, it is not necessary to expand the liquid 34 to a pressure above the inlet pressure of the compressor C1 therefore, in this case, the vaporized liquid is sent to a bypass valve 46 that does not expand it substantially or at all.

The rest of the liquid 13 is expanded in a valve 19 (or a liquid turbine), without going through the exchanger E1, and sent to a phase separator P1. In the phase separator, it forms a liquid 23 and a gas 21. The liquid 23 is heated in the exchanger then sent to the third phase separator P3. The liquid of this separator is the carbon dioxide-rich product 25 of the apparatus, at −50° C. and 7 bar abs. The gases of the separators P1 and P3 are mixed, cooled in the exchanger E1, compressed by a compressor C5, cooled in a cooler 31, then heated in the exchanger E1 and sent to be sent to the second phase separator P2. The gas 33 of this separator contains 30% hydrogen, 50% carbon dioxide and 15% nitrogen and is heated in the exchanger E1. The liquid is heated in the exchanger as the flow 36, then mixed with the fraction 13 sent to the separator P1.

It is obvious that the process does not necessarily include the treatment of the fraction 13. In this case, the liquid 20 constitutes one of the liquid products of the process.

It is also possible to treat the fraction 13 by distillation in order to produce a carbon dioxide-rich liquid product or treat it very simply in order to remove a gaseous portion, the portion 22 comprising a liquid product. For example, a mercury and/or NOx elimination column can be provided in place of or in addition to a nitrogen or oxygen elimination column.

It may be envisaged that the process is fed using a single source and that the composition of the feed gas varies over time. In this case, it will be necessary to adjust the pressure up to which the valve 43 expands the liquid 34. Thus, the inlet pressure of the valve 45 will also be modified, but the outlet pressure of the valve 45 will remain constant.

It is also possible that the origin of the feed gases varies over time, since a feed gas with a different composition begins to feed the process during operation. In this case, it is necessary to be capable of taking into account a possible composition with an increased content of light impurities during the operation of the process.

It will be understood that, expressed more simply, the process requires only vaporizing the flow 34 at the lowest pressure. The other vaporizations at higher pressures make it possible to improve the exchange diagram, but are not essential.

According to FIG. 2, if the rate of light impurities in the flow 3 is high, then the presence of the so-called output compressor C5 in the system for treatment of the fluid 13 of FIG. 1 becomes particularly advantageous. In this case, the valve 45 of FIG. 1 can be replaced by a turbine that is coupled to the compressor C5.

In this figure, a portion 1 of the feed gas is already at a high enough pressure to be liquefied in the exchanger E1. Two flows 1A, 1B at the first pressure are therefore compressed up to the second pressure by being sent to the third stage of the compressor, as described above, and the other flow 1 at the second pressure is sent directly to the exchanger E1, where it is at least partially liquefied before being sent to the phase separator. It will be noted that the flow 1 can be the only flow treated in the phase separator or, otherwise, that it can be mixed with the fraction 13. This procedure also makes it possible to manage the case in which the flow 1 and the flows 1A, 1B have very different purities. For example, the flow 1 can be sent either to the separator P1 or to the separator P2 or to the separator P3 according to its composition.

By contrast with FIG. 1, the head gases of the phase separators P1, P2 are compressed by a compressor C5, cooled in the cooler 31, then compressed by a compressor C6 and then cooled in the cooler 32 before being cooled in the exchanger E1 upstream of the separator P2. The turbine 45 is coupled to the compressor C5 or to the compressor C6.

The regulation is performed as for FIG. 1, except that the expansion rate of the turbine 46 is adjusted as a function of the pressure of the flow 41.

It is possible to couple the turbine 45 with the compressor C1.

While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.

“Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.

“Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary a range is expressed, it is to be understood that another embodiment is from the one.

Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such particular value and/or to the other particular value, along with all combinations within said range.

All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited. 

1-14. (canceled)
 15. A process for liquefying a gas containing at least 60 mol % of carbon dioxide and at least one impurity lighter than carbon dioxide, in order to produce at least one liquid product wherein a first feed gas containing at least 60 mol % of carbon dioxide and at least one light impurity at a feed pressure is cooled in order to form a liquid or supercritical flow, at least a portion of the liquid or supercritical flow is cooled in a heat exchanger to form a cycle fluid having a cycle pressure, the cycle fluid is divided into at least two fractions comprising a first fraction and an auxiliary fraction, the first fraction being expanded down to a first pressure in a valve in order to form a biphasic mixture and then sent to a phase separator, the liquid fraction of the phase separator is vaporized so as to form a vaporized gas in the exchanger, the vaporized gas then being expanded from the first pressure to a second pressure in an expansion means and then compressed in a cycle compressor and mixed with the first feed gas, the auxiliary fraction either comprising the liquid product or one of the liquid products or being treated by separation at sub-ambient temperature in at least one separation means to form the liquid product or one of the liquid products, and wherein the first pressure is varied as a function of the amount of impurities lighter than the carbon dioxide contained in the carbon dioxide-rich gas, wherein the first pressure is varied so that the higher the concentration of light impurities is, the higher the first pressure is.
 16. The process of claim 15, wherein the first feed gas is cooled so as to form a liquid or supercritical flow, after having been compressed in a cycle compressor.
 17. The process of claim 15, wherein the second pressure is substantially constant and independent of the composition of the carbon dioxide-rich gas.
 18. The process of claim 15, wherein the second pressure is equal to the inlet pressure of the cycle compressor and the gas expanded in the expansion means is sent directly from the expansion means to the inlet of the cycle compressor.
 19. The process of claim 15, wherein the first pressure is chosen so as to be below the pressure at which the carbon dioxide-rich gas would solidify in the phase separator or in the valve.
 20. The process of claim 15, wherein the expansion rate in the expansion means is regulated as a function of the composition of the gas to be expanded in the expansion means and/or as a function of the composition of the fluid expanded in the expansion valve.
 21. The process of claim 15, wherein the expansion rate in the expansion means is regulated as a function of the amount of light impurities in the gas to be expanded in the expansion means and/or as a function of the composition of the fluid expanded in the expansion valve.
 22. An apparatus for liquefying a gas containing at least 60 mol % of CO₂ and at least one impurity lighter than carbon dioxide to produce at least one liquid product, comprising a heat exchanger, a cycle compressor, a cooling means, a conduit for sending a feed gas containing at least 60 mol % of CO₂ and at least one light impurity to the cooling means in order to form a liquid or a supercritical fluid, a conduit for sending the liquid or supercritical fluid to the exchanger in order to form a cooled fluid, a conduit for sending a portion of the cooled fluid as an auxiliary fraction to a client or to a treatment, an expansion valve, a conduit for sending a portion of the cooled fluid to the expansion valve, a phase separator, a conduit for sending a fluid expanded in the expansion valve to the phase separator, a conduit for sending the liquid from the phase separator to the exchanger, expansion means, a conduit for sending the liquid vaporized in the exchanger to the expansion means to form an expanded gas, a conduit for sending the expanded gas to a cycle compressor, means for mixing the expanded gas with the gas containing at least 60 mol % of CO₂ upstream of, downstream of, or in the cycle compressor and means for varying the outlet pressure of the expansion valve as a function of the composition of the feed gas, wherein the expansion means is a turbine.
 23. The apparatus of claim 22, comprising a treatment unit connected to the conduit in order to send a portion of the cooled fluid as an auxiliary fraction to a treatment, the treatment unit being a liquefaction unit or a separation unit at sub-ambient temperature.
 24. The apparatus of claim 22, wherein the turbine is coupled to the cycle compressor.
 25. The apparatus of claim 22, wherein the treatment unit comprises a compressor and the turbine is coupled to the compressor of the treatment unit.
 26. The apparatus of claim 22, comprising a means for detecting the concentration of the gas sent to the expansion means and/or the fluid sent to the expansion valve and a means for varying the expansion rate of the expansion means as a function of this concentration.
 27. The apparatus of claim 22, comprising a means for detecting the pressure of the gas sent to the expansion means and a means for varying the expansion rate in the expansion means as a function of this pressure.
 28. The apparatus of claim 22, wherein the exchanger is arranged so that the latent heat from vaporization of the liquid of the phase separator is transferred to the liquid or to the supercritical fluid that is cooled through the exchanger. 